1. Field of Invention
The present invention relates in general to a Power Headroom reporting mechanism, and more particularly to a Power Headroom reporting mechanism for radio resources allocation in a wireless communication system.
2. Related Art
In a wireless communication system, how to efficiently distribute/allocate uplink transmission resources between multiple user equipments (UEs) is always a critical issue from the system point of view. In order to achieve better radio resource management, some mechanisms which is used for enabling UEs to provide related information and informing the network side of some UE-specific power status, are usually introduced in the wireless communication system. For example, procedures called Power Headroom reporting are introduced in the 3rd Generation Partnership Project (3GPP) Universal Terrestrial Radio Access (UTRA) system and Evolved UTRA (E-UTRA) system. Also a mechanism called Power Status Reporting is introduced in IEEE 802.16m.
Taking the Power Headroom reporting procedure in 3GPP E-UTRA for example, the Power Headroom is the difference between the nominal UE maximum transmit power and the estimated power for UpLink-Shared Channel (UL-SCH) transmission. And the reporting procedure is introduced to report the difference to the network side, so that the network entity (eNB) can perform radio resource management in an appropriate way. This is especially important when allocating the transmission format (bandwidth and modulation and coding scheme) to different UEs. The network entity (eNB) can use the Power Headroom value to determine how much more uplink bandwidth per subframe which a UE is capable of using. This can help to avoid allocating uplink transmission resources to UEs which are unable to use them, thus a Signal to Interference-plus-Noise Ratio (SINR) is decreased and the system capacity may be wasted.
The behavior of the Power Headroom reporting procedure is defined in 3GPP protocol specifications. In the 3GPP protocol specifications, a Power Headroom Report (PHR) shall be triggered if any of the following events occur:
prohibitPHR-Timer expires or has expired and the path loss has changed more than dl-PathlossChange dB since the last transmission of a PHR when the UE has uplink resources for a new transmission, wherein only one component carrier is operated by the above-mentioned UE, and the above-mentioned path loss belongs to the component carrier of the UE.
periodicPHR-Timer expires; and
upon configuration or reconfiguration of the power headroom reporting functionality by upper layers, which is not used to disable the function.
A possible implementation example of the above described PHR triggering mechanism in the 3GPP protocol specifications is illustrated in FIG. 1. FIG. 1 is a flowchart of a conventional PHR triggering method according to the 3GPP specifications. However, the triggered PHR is only for one component carrier.
In the 3GPP protocol specifications, it is also read a Power headroom reporting procedure to enable the UE to transmit the PHR to the network side. FIG. 2 illustrates a flowchart of a conventional Power headroom reporting method according to the 3GPP specifications. Referring to FIG. 2, the method includes the following steps.
In step S201, the method for reporting a PHR starts.
In step S202, it is determined whether the UE has uplink (UL) resources allocated for a new transmission for the present Transmission Time Interval (TTI).
In step S203, it is determined whether it is the first uplink (UL) resource for the new transmission since the last Medium Access Control (MAC) reset.
In step S204, a periodicPHR-Timer is started.
In step S205, it is determined whether the PHR has been triggered since the last transmission of a PHR or whether it is the first time that a PHR is triggered.
In step S206, it is determined whether the allocated uplink (UL) resources can accommodate a PHR MAC control element plus its sub-header after the logical channel prioritization.
In step S207, the value of the power headroom is obtained from the physical layer.
In step S208, the multiplexing and assembly procedure are instructed to generate and transmit a PHR MAC control element based on the value reported by the physical layer.
In step S209, the periodicPHR-Timer is started or restarted.
In step S210, a prohibitPHR-Timer is started or restarted.
In step S211, all triggered PHR(s) is/are cancelled.
In step S212, the method for reporting a PHR ends.
The above PHR triggering and reporting mechanisms are designed for release 8 and release 9 (Rel-8/Rel-9) 3GPP E-UTRA system, which is commonly known as 3GPP Long Term Evolution (LTE), but as new features are introduced in the future release system, which is commonly known as 3GPP LTE-Advanced, the PHR triggering and reporting mechanisms defined in Rel-8/Rel-9 may be insufficient. For example, the UE may be configured with simultaneous transmission of Physical Uplink Control CHannel (PUCCH) and Physical Uplink Shared CHannel (PUSCH) on the same carrier in LTE-Advanced, while no such simultaneous transmission is supported in Rel-8/Rel-9 E-UTRA system. Another newly introduced feature for LTE-Advanced is the Carrier Aggregation (CA). A UE with reception and/or transmission capabilities for CA can simultaneously receive and/or transmit on multiple component carriers (CC) corresponding to multiple serving cells. The introduction of transmission on multiple component carriers may also bring the need of new PHR mechanism.